There's no overnight shortcut, it does take some consistent practice to see improvements. You have to make a start NOW, from whatever your current position, you can't spend your life waiting for everything to fall into place so you can do the things you want, just start, start small and be consistent. I wasn't fat or particularly unhealthy when I started but I was basically adapted to climbing and doing bodyweight exercises with additional weight strapped to me.
So what was my 3rd time running 100 miles I went to Spain for Ultima Frontera 160km. By now I'd been running regularly for 9 months and sporadically for the same again before that.
Once I decided to take this running thing seriously I started to train 3-4 runs per week on top of the bodyweight training I was doing I was eating a colossal amount to keep my energy up and was still able to gain muscle mass despite running or cycling daily. I didn't try and run every day or set any goal that was too outlandish (maybe it depends who you ask) but I decided that running every other day was achievable and I kept at it.
Anyhow so October 2011 I'd done a handful of ultras quite a few marathons or similar training runs and trained well through the year, now it was time to travel to a different country to race in the heat of Spain and in some decent mountains. I had made a lot of the mistakes in smaller races but I'd kept my training up throughout all the challenges of everyday life, roughly I did the following in a typical month
weekend 1 - ultra
weekend 2 - long training run
weekend 3 - back to back long runs
weekend 4 - some cycling and a short race 10k/ half marathon
Through the week I'd do about 3 runs,
Run 1 around 90 mins to 2 hours might be 10 mile or 2 x 6 mile depending,
Run 2 faster session, either progressive or intervals or something with some speed
Run 3 - Easy effort could be hills or just local trails
Along with this I was cycling on the days I didn't run mainly to get around, no structured sessions on the bike and training in the gym with bodyweight and kettlebells or going climbing about 5 days per week (This is probably a little high but I was doing both and hadn't picked a main sport at this point)
So vaguely that's what I'd do.... I didn't always feel fresh and ready but I pushed myself and got it done.
When it came to the actual race... That's where I executed #3 ...